The object of the invention is a process for the continuous preparation of sodium aluminosilicates that have the smallest particle size, still contain bound water and have a proportion of less than 0.1% by weight of material with a grain size exceeding 25 .mu.m, on mixing sodium aluminate dissolved in water with sodium silicate dissolved in water, in the presence of excess sodium hydroxide solution.
Alkali metal aluminosilicates are characterized in the conventional way of describing oxides by the general empirical formula EQU xKat.sub.2/n O.Al.sub.2 O.sub.3.ySiO.sub.2.zH.sub.2 O.
In this oxide formula, Kat is a cation of the valency n, which is exchangeable with other cations, such as an alkali metal, x is a number from 0.2 to 1.5 and preferably about 1, y is a number exceeding 1.5. The content of water depends on the degree of drying. The above formula includes a large number of substances that frequently differ only insignificantly in their chemical composition but considerably with respect to their structure and properties. The x-ray diffraction diagram usually is employed for the identification of crystalline types, in addition to the chemical composition.
Technically interesting are mainly those synthetic alkali metal aluminosilicates that possess a three-dimensionally cross-linked aluminosilicate lattice. In the anionic lattice formed from SiO.sub.2, part of the silicon(IV)-atoms is replaced by aluminum(III)-atoms; the missing charge is balanced with cations--one univalent cation each per aluminum atom in the lattice. The so-called zeolites form a mineral class of alkali metal aluminosilicates containing water of crystallization with defined pore and spatial structure of their aluminosilicate lattice. "Molecular sieves" are only those zeolites that are used technically for the separation of materials because of these lattice properties. Synthetic zeolites are increasingly gaining in technical importance and are utilized as cation exchange agents mainly for the softening of water, as catalysts in chemical processes, as drying, separating or adsorption agents for solvents and gases and as heterogeneous inorganic builders in washing and cleaning agents. Depending on the purpose of application, different types, having different degrees of dryness and purity are needed. Normally, such molecular sieves initially are prepared in their sodium form and subsequently converted into other forms by ion exchange. For the technically important molecular sieve types A and X, which are often labelled differently in the literature, the terms NaA and NaX, respectively, are employed, for example, for the sodium forms, and the terms KA and KX, respectively, are employed for the potassium forms.
The two molecular sieve types NaA and NaX have special technical significance. The chemical composition of the molecular sieves of the type NaA corresponds to the molecular formula EQU 1.+-.0.2Na.sub.2 O.1Al.sub.2 O.sub.3.2.+-.0.5SiO.sub.2.0 to 6H.sub.2 O,
that of the molecular sieve type NaX, which is richer in silicate, to the formula EQU 0.9.+-.0.2Na.sub.2 O.1Al.sub.2 O.sub.3.2.5.+-.0.5SiO.sub.2.0 to 8H.sub.2 O.
The x-ray diffraction diagram of molecular sieve NaA is described, for example, in the German published patent applications DE-AS No. 10 38 015 and DE-AS No. 10 38 017, that of molecular sieve NaX, in DE-AS No. 10 38 016, corresponding to U.S. Pat. No. 2,882,244.
An additional zeolitic sodium aluminosilicate with increasing technical importance is the cubic molecular sieve P, which is richer in silicate. This zeolite is also known as "Zeolite P.sub.c " or "Molecular Sieve B". The chemical composition of molecular sieve P corresponds to the molecular formula EQU 0.9.+-.0.2Na.sub.2 O.1Al.sub.2 O.sub.3.4.+-.1.3SiO.sub.2.0 to 6H.sub.2 O.
The x-ray diffraction diagram is shown, for example, in D. W. Breck, "Zeolite Molecular Sieves", New York 1974, page 365.
The x-ray diffraction diagram of hydrosodalite ("Zeolite HS") is also found there (page 360). This sodium aluminosilicate also has a lattice structure and chemical composition similar to the type NaA, but it is not usually included among the zeolites since a corresponding pore structure is missing. The hydrosodalite is an undesirable byproduct in many molecular sieve syntheses since it has a weaker cation exchange capacity.
These and other sodium aluminosilicates of various types are produced technically mainly in such a manner that the aluminate and silicate components dissolved completely and preferably with the use of excess sodium hydroxide solution, are combined and thoroughly mixed with vigorous agitation above room temperature, while maintaining certain molar and concentration ratios in the batch. The initially forming aluminosilicate gel is broken up by the application of strong shearing forces into an agitatable sol. The resulting x-ray-amorphous sodium aluminosilicate precipitates are subsequently crystallized under defined crystallization conditions. Depending on the purpose for which they are to be used, these sodium aluminosilicates are filtered, freed from excess alkali and dried. Several forms of this hydrothermal synthesis are listed in D. W. Breck (cf. above), others are described, for example, in the German published patent application disclosures DE-OS Nos. 16 67 620, 20 28 163, 22 00 745, 23 05 993, 23 33 068, 25 14 399, 26 05 083 and 26 05 113. Examples for the preparation of x-ray-amorphous sodium aluminosilicates for special purposes are found in the German published patent application disclosures DE-OS Nos. 17 17 158 and 25 49 659.
The distribution of the particle size of the utilized sodium aluminosilicate powder is very important for most purposes, particularly when it is employed in aqueous suspension, for example, for adsorption and exchange properties, sedimentation rate, abrasivity as well as the behavior of the residue on smooth surfaces or textiles. The smallest possible powder particles are desired in the majority of these cases. An additional requirement for use in washing, rinsing and cleaning agents, for example, is the absence of any measurable content of particles exceeding 25 .mu.m. This content, which is determined according to DIN 53 580 by wet screening according to Mocker, is called grit.
So far, only few data providing information in figures concerning the granule size distribution of sodium aluminosilicates are known from the literature. Most of the literature references refer to the technically important molecular sieve NaA. A characteristic distribution curve for type NaA (="4A") is found in D. W. Breck, "Zeolite Molecular Sieves", New York 1974, page 388. According to this, the major proportion of the particles lies below 4 .mu.m. However, considerably larger crystal particles than those of the similar molecular sieve types NaA and NaX are found in molecular sieve P, with particle sizes normally above 5 .mu.m (cf. Taylor and Roy, "The American Mineralogist", vol. 49, 1964, page 662). These particle sizes were determined by electron microscope, however.
Suspensions of previously dried sodium aluminosilicates in powder form may contain considerably larger particles, however. Conventional methods for the determination of the particle size distribution are based either on the different sedimentation of particles of different size (sedimentation analysis) or on the electronic measuring of the volumes of the particles suspended in a test electrolyte by means of the disturbance of an electrical field in the counter aperture (counting by Coulter-Counter.RTM.).
Processes for the preparation of grit-less, zeolite powder of type A, consisting of fine particles and having defined particle size spectra and contents of 50% by weight of particles smaller than 4.9 .mu.m (or 6.2 .mu.m, 4.3 .mu.m, 5.9 .mu.m and 4.0 .mu.m, respectively), are described in the German published patent application disclosures DE-OS Nos. 26 51 419, 26 51 420, 26 51 436, 26 51 437 and 26 51 485. In German published patent application disclosure DE-OS No. 25 14 399, the preparation of a low-grit zeolite molecular sieve powder with a mean granule diameter below 10 .mu.m, determined by sedimentation analysis, is described. Here the aqueous medium surrounding the crystallization product is adjusted to a pH between 8.5 and 11 before drying.